Numerous applications for measuring the moisture content of soils, grains, lumber, and industrial process products exist. A complete sensing system consists of the circuitry and also a probe containing at least two conducting surfaces electrically insulated from each other to form a capacitive structure which can be inserted into the material to be measured. The circuitry is electrically connected to the probe so that an alternating current (AC) stimulus can be applied to one of the probe surfaces while the other surface remains grounded. The circuit output can be used to determine the capacitance and resistance of the sample between the conducting surfaces, which in turn, can be related to the sample conductivity and dielectric constant. Using well know relationships, the sample's moisture content and salinity can be ascertained.
Typically, moisture sensing devices in the past have included a container into which the material is placed, with plates or the like therein for determining the capacitance of the material placed therein and relating that capacitance to the moisture content. For example, the U.S. Pat. No. 3,209,247 to Mead and the U.S. Pat. No. 4,050,016, to Marsh et al show typical devices of this sort. These devices are, however, inconvenient to use since they require removing a portion of the material to be tested. Further, removing the material, for example, by digging a sample of soil, necessarily changes its density so that the measured results are not necessarily the actual moisture content of the soil before its removal.
Another inaccuracy arises in many of these devices because they measure only the capacitance of the soil or measure only the resistance. U.S. Pat. No. 3,803,570 to Barrow et al, for example, describes a capacitance measuring device. None of these capacitance, devices however, have effectively combined high accuracy with ease of use. U.S. Pat. No. 2,870,404 to Oxley describes a resistive measuring device in which a plurality of spikes are inserted into the ground. In fact, both the resistance and capacitance of the soil vary with moisture and vary independently of each other depending upon soil condition. The relation of resistance to moisture particularly is non-linear and very difficult to predict for any given composition. Devices which ignore variation of resistance with capacitance necessarily produce an inaccurate indication of moisture content.
U.S. Pat. No. 4,288,742 to Walsh discloses a unique, simple, and effective moisture sensor which can be inserted easily into material to be measured, usually without damage to that material, and which takes into account both resistance and capacitance to produce an accurate indication of moisture content. The sensor includes a probe having at least a single, and preferably a plurality of spines extending outward from a base so that the spines can be inserted into the material. The spines are sufficient in number to appear as a ground plane forming an effective coaxial capacitor. Inaccuracies resulting from fringing fields are eliminated while the device remains easily insertable.
The impedance produced by the material surrounding the spines forms part of an RC bridge, preferably a Wien or other bridge, which also includes a separate resistor and capacitor. Thus, the impedance of the material, both its capacitance and resistance, are measured to produce signals indicating that impedance. By determining the ratio of the voltages across the RC circuit forming part of the bridge and the RC circuitry of the material impedance and determining the resonant frequency, both the resistance and capacitance of the soil can be determined and related to the dielectric constant of the material. From that dielectric constant the soil moisture content can be easily determined according to well known relations.
The coaxial geometry of the device accurately defines the active volume by minimizing fringe volumes. With sensors of the type which use plates, the fringe capacitances introduce errors since those capacitances vary with the dielectric constant. The coaxial geometry has no such fringe capacitance, except at the ends. A first ring of spines extend outwardly from a base in parallel with a second ring of spines extending outwardly from the base, also in parallel, and within the first ring, separated and insulated electrically therefrom. The two rings thus form an effective coaxial capacitor which can be inserted into the material to be sensed.
U.S. Pat. No. 4,540,936, also to Walsh, describes an improved moisture sensor. In one embodiment described in the Walsh patent, two slotted cylindrical tubes are mounted coaxially and replace the spines described in the above mentioned application. The cylindrical tubes are mounted coaxially and replace the spines described in the above mentioned application. The cylinders are sharpened on the end which pushes into the ground. A simple insulating plug is used to mount and electrically separate the two sensors.
In a second embodiment the volume is partially bounded by a cross shaped member having flat surfaces defining the volume in cross section as a square center with a rectangular leg extending from each side thereof. Each leg is open at the peripheral edge. The member and the volume in the legs tapers in the longitudinal direction so that at the insertion end the volume is made up only of the center section. A plurality of parallel plate capacitors are thereby formed by the parallel facing surfaces which are driven at the same potential. The four legs provide not only a controlled volume but good mechanical rigidity. The outer part of each leg can be insulated from the rest of the member is desired to serve as a guard ring.
In a third embodiment two cross-shaped members are jointed together at the peripheral edge of one leg of each. This gives better definition of the electrical volume since more of the volume is remote from openings and therefore less susceptible to fringe effects.
The sensors described by Walsh in U.S. Pat. No. 4,540,936, however, have several disadvantages. One of the described circuits requires variable resistors and capacitors, a variable frequency oscillator, a frequency detector, and an AC meter. These components add considerably to the cost, size, and complexity of the circuit. During the measurement process, the variable frequency oscillator must be swept through a range of frequencies in order to determine the resonant frequency. In general, the variable resistor and capacitor also must be adjusted during the measurement process. This makes the measurement process very complicated.
Another of the Walsh embodiment avoids the problems and difficulties associated with using frequency detectors, variable frequency oscillators, variable resistors and capacitors. However, it requires the use of an operational amplifier which is difficult at frequencies much in excess of 1 MHz as well as synchronous switches and a 90 degree phase shifter. In practice, considerable difficulty is involved in properly tuning the synchronous switches and in ensuring that the 90 degree phase shiftier produces a precise 90 degree phase shift. These components are also generally expensive and large. Finally, all three outputs, vout, er, and ei are AC voltages requiring AC meters.
It is accordingly an object of the present invention to provide an improved soil moisture sensor having circuitry which avoids the deficiencies of the Walsh patent as well as other prior art.
It is a further object of the invention to provide improved circuitry for a soil moisture sensor which is more compact and less expensive than previous designs.